If nothing else, the prince really has a knack for finding gullible, easily manipulated people who are willing to throw their future away just to get noticed by the prince. And he's not even the first prince, just the third prince. Of course we don't actually know how the royal inheritance works in that kingdom. Maybe all the princes are acting like utter scumbags because they were tasked to do so by the king, to prove they can shamelessly and emotionlessly use people. Then the most successful manipulator will be made the crown prince. However, seeing how poor the writing is in this series, I doubt the author has the skills to write any intelligent motivation behind the prince's actions. He's probably just bored and doing this for his own amusement.